DESCRIPTION (Applicant's Abstract): The long-term goal of the proposed research is to identify and characterize the mechanisms that organize and pattern tissues during development. Specifically, we study the creation and function of the compartment boundaries and developmental organizers in Drosophila imaginal discs, and focus on the mechanisms that enable cells in the discs to communicate over long distances. During previous granting periods, we identified and characterized several of the key players that set up and regulate compartment border and organizers - Engrailed, Hedgehog and Cubitus interruptus. During the most recent granting period, we discovered long cytoplasmic extensions (cytonemes) that project from outlying cells in imaginal discs to the organizers. We have shown that these novel structures are common to many different vertebrate and invertebrate cell types and we have offered the exciting possibility that cytonemes are responsible for distributing signals over long distances in a variety of different contexts. Specific objectives proposed here to further characterize the mechanisms responsible for cell-cell communication between developmental organizers and their target cells are: 1. Functional and physical characterization of cytonemes and the factors that induce their growth. 2. Elucidation of the mechanism of Hedgehog signaling.